I've used him. I used to play quite a bit of NU, and even then he was bah.
Bastiodon has high Defense and Special Defense, yes. However, he has a fairly low HP stat, lacks any form of reliable recovery, and Steel-Rock is a pretty meh typing, with 4x weaknesses to common attacking types like Ground and Fighting. Non-existant stats on offence means that outside of Toxic/Metal Burst, Bastiodon finds it difficult to deal any sort of meaningful damage. Bastiodon's most common set is Toxic / Stealth Rock / Roar / Magic Coat or Metal Burst. It's for those reasons that it's in PU this gen, the worst possible tier for Pokemon which cannot evolve further.
Skarmory on the other hand, functions as a much better physical wall . It has a lower defense, yes, but base 140 is more than enough, and it has a slightly higher HP stat. It most importantly, has access to Roost which provides reliable recovery, and has a better typing in Steel/Flying which provides a neutrality to Fighting and an immunity to Ground. Base 80 Atk + Brave Bird which is a powerful move in its own right, coupled with STAB means Skarmory can still deal somewhat usable damage. Skarmory can also opt to run Defog, which is one of the most useful moves for a team support this gen, removing all forms of hazards as well as Light Screen and Reflect.
TL:DR useless compared to skarmory
--------------------------------------------------------------------------------------------------------------------
Fun huh? Yeah, I find it so fun getting trashed by every second decent team out there because I'm using PU in OU or something.
Fun is fine, but in my mind, fun =/= unviable.